This invention relates to the transfer of toner images. More specifically, it relates to an image forming apparatus in which the transfer of a toner image to a receiving surface, either of a receiving sheet or an intermediate member is controlled in varying ambient conditions.
U.S. Pat. No. 3,781,105 to Meagher, issued Dec. 25, 1973, suggests a multilayered transfer backing roller having intermediate conductivity. More recently, typical backing members having a polyurethane blanket with a resistivity of 1.times.10.sup.9 ohm-cm at 70.degree. F. and 50 percent relative humidity are used to control ionization in both the pre-nip and post-nip regions.
However, the resistivity of intermediate conductivity materials is not stable over a typical range of operating temperatures and humidities and also changes somewhat as the roller ages. Variations in the resistivity of rollers alter the response time of the transfer subsystem, i.e., the rate at which the electric field increases and decreases as the image traverses the nip. Such variation results in a less robust subsystem because of image quality degradation in the form of ionization defects and reduced transfer efficiency. Although somewhat different in degree, these effects exist both with conventional transfer directly to a receiving sheet backed by a transfer member of intermediate conductivity and with transfer directly to a transfer member that functions as an intermediate. Although rollers are typically used as both backing members and intermediate members, belts and other configurations for such transfer members are also used.
U.S. Pat. No. 5,084,737 to Hagen et al also notes that an increase in humidity causes moisture to be absorbed by polyurethane or other material substance being used for the intermediate conductivity layer (blanket) on a transfer drum. It notes that the electrical properties of the structure between the core of the polyurethane layer and the conductive backing on the original image member can be modeled by a simple RC circuit. The time required for the transfer field to reach full application varies according to the resistance of the polyurethane layer. In this particular instance, the transfer field was applied only after the initial portion of a receiving sheet is held by a vacuum. The response time of the system, thus, is critical. The solution to varying conditions in this reference is to vary the time of application of the field as a function of the relative humidity or the resistance of the transfer drum. A constant current source is used to create the transfer field. Relative humidity is determined for use in adjusting the field by sensing the voltage applied by the constant current source. See also, U.S. Pat. No. 5,036,360 to J. F. Paxon et al, issued Jul. 30, 1991.
U.S. Pat. No. 4,014,605, issued to Fletcher Mar. 29, 1977, is one of a number of references that suggests use of an erase lamp during transfer when one of the components in the transfer process includes a photoconductive layer. It especially suggests erasing in the post-nip region. See also, U.S. Pats. 3,734,724 to York, issued May 22, 1973; 3,707,138 to Cartwright, issued Dec. 26, 1972; and 3,684,362 to Weigl, issued Aug. 15, 1972.